The present invention relates to a brake system with electronic anti-lock control for automotive vehicles with all-wheel drive and variable coupling of the drive between front axle and rear axle.
Brake systems with electronic anti-lock control are known whose principle necessitates the use of four, or at least three, wheel sensors. This is because a reference variable, the so-termed vehicle reference speed, is required for control of the brake slip. The reference variable is formed by logic combining of the individual measured values of the rotational speed of all wheels or of at least the front wheels and the rear axle and which is in all situations only `correct` or apt for the control if measured values of all wheels are available. For example, according to German laid-open print 22 54 295 the formation of the reference variable makes a difference between driven and non-driven wheels.
In vehicles with all-wheel drive, however, the formation of this reference variable is considerably complicated by the coupling of all wheels by way of the drive shafts, by the transmission of drive torques and brake torques caused thereby as well as by the high moments of inertia at all wheels. The fixed coupling of the wheels among each other has a consequence that the wheels mutually influence each other, for which reason variations of the brake or friction force between tires and road can no longer be recognized definitely by measurement of the rotational behavior of the individual wheels and logic combining of the measured values. In many situations, coupling by way of the drive results in an almost synchronous behavior of all wheels, which renders it difficult to detect instability or an imminent locked condition. After engagement of the differential locks, these problems are even more pronounced.
Various measures to lessen these difficulties are known. In all cases, major efforts are required. At least, there is always need for an individual measurement of the rotational speed at all wheels. Simplified anti-lock systems with only two wheel sensors and merely two control channels fail to provide an effective solution.
A known brake system for all-wheel driven vehicles, provides for automatic de-activation of the anti-lock control when differential locks are engaged. This is a serious disadvantage because unfavorable road conditions necessitate both the differential locks and the anti-lock control to preserve drive stability.
Therefore, it is an object of the present invention to overcome the described shortcomings of the known anti-lock control systems and to provide a slip-controlled brake system for all-wheel driven vehicles which, in any situation, prevents locking of the wheels and ensures efficient braking at a short stopping distance and which, nevertheless, distinguishes by particularly little manufacturing effort.